Process for recovering low-sulfate bischofite

ABSTRACT

Aqueous brines containing both magnesium chloride and magnesium sulfate are treated to recover a low-sulfate bischofite (MgCl2.6H2O). The brines are subjected to evaporation, as required, to produce a brine which is substantially supersaturated with respect to magnesium sulfate. The evaporation is conducted at high temperatures to avoid crystallization of magnesium chloride. The supersaturated magnesium sulfate brine is then held at high temperature under conditions which maintain the magnesium chloride in solution for a sufficiently long duration to permit the crystallization of magnesium sulfate as MgSO4.5/4 H2O. Clarified brine recovered from the resulting sulfate-crystal slurry is subjected to evaporation under vacuum and at low temperature, preferably below about 90* C., at sufficient rate to produce a low-sulfate bischofite crystal crop.

United States Patent Hahn et a1.

1 5] Feb. 15, 1972 [541 PROCESS FOR RECOVERING LOW- SULFATE BISCHOFITE[72] Inventors: Burkhard .1. Hahn, Ronnenberg; Rolf M. E. Reise; WalterR. Raschka, both of Bad Salzdetfurth, all of Germany [73] Assignee:Salzdetfurth A.G., Hannover, Germany [22] Filed: Apr. 12, 1968 [21]Appl. No.: 720,959

[52] US. Cl ..23/304, 23/91 2,895,794 7/1959 Dancy ..23/128 3,101,2478/1963 Serowy ..23/l2 8 3,499,725 3/1970 Scarfi et a1.. ..23/1283,342,548 9/1967 Macey ..23/9l OTHER PUBLICATIONS Vol. 4, Inorganic andTheoretical Chemistry, Mellor, pp.

fiimaryExtzminer-Norman Yudkofi' Assistant Examiner-S. SilverbergAttorney-David V. Trask ABSTRACT Aqueous brines containing bothmagnesium chloride and magnesium sulfate are treated to recover alow-sulfate bischofite (MgCl -6H O). The brines are subjected toevaporation, as required, to produce a brine which is substantiallysupersaturated with respect to magnesium sulfate The evaporation isconducted at high temperatures to avoid crystallization of magnesiumchloride. The supersaturated vmagnesium sulfate brine is then held athigh temperature under conditions which maintain the magnesium chloridein solution for a sufficiently long duration to permit thecrystallizationof magnesium sulfate as MgSO -5/4 H O. Clarified brinerecovered from the resulting sulfate-crystal slurry is subjected toevaporation under vacuum and at low temperature,

preferably below about 90 C., at sufficient rate to produce alow-sulfate bischofite crystal crop.

14 Claims, 1 Drawing Figure EVAPORATION MgCI BRINE SUPERSATURATED WITHRERECT TO MgSO HOLDING VESSEL SLURRY OF M so -74 H2O CLARIFIED BRINESTEAM VACUUM EVAPORATION I SLURRY or M 0, eu o SLURRY OF MgCI EH 0CLARIFYING AND vDECANTING CONCCEFNTRATED SLURRY BISCHOFITE PRODUCT IRESIDUAL BRINE STEAM VACUUM EVAPORATION II SATURATED MQCI BRINE SLURRYOF MgC| -6H O MOTH LIQUOR BISCHOFITE PRODUCT II PAIENIEIIIEBIB Ian I3.642.455

BITTERNS STEAM L EVAPORATION T MgClg BRINE SUPERSATURATED WITH REISECTTO M9804 HOLDING VESSEL T SLURRY OF MgSO IH O CONCENTRATED SLURRY igimOF M so H2O VACUUM EVAPORATION I SLURRY OF MgCl 6H O COOLING SLURRY OFMgCl 6H O FILTERING L T RESIDUAL BRINE STEAM BISCHOFITE PRODUCT I TVACUUM EVAPORATION II SATURATED MgCl BRINE COOLING SLURRY OF M cI -6H2OFILTERING BISCHOFITE PRODUCT ]I M INVENTORS RKHARD J. HAHN LF E. REI ER. RAS HKA ATTORNE YS PROCESS FOR RECOVERING LOW-SULFATE BISCHOFITEBACKGROUND OF THE INVENTION Field:

This invention relates to the production of magnesium chloride frombrine. Specifically, it provides a method for producing low-sulfatebischofite from brines supersaturated with respect to magnesium sulfate.

State of the Art:

It is known to evaporate brines which contain both magnesium chlorideand magnesium sulfate, such as bitterns, to obtain a bischofite product.Such products normally contain high levels of sulfate impurities. Theproduction of low-sulfate bischofite from brine containing even smallamounts of sulfate impurities has involved complicated techniques.

Heretofore, low-sulfate bischofite products have been obtained from suchbrines by chemically treating the brine. Thus, for example, brines havebeen treated with calcium chloride and barium chloride to remove sulfateprior to subjecting the brines to evaporation. An alternate procedureinvolves removing sulfate by first evaporating to crystallize artificialkieserite (MgSO,,-/4ll.,0) and then treating the hot MgCl solution withcalcium chloride and barium chloride to remove the residual sulfate.Other procedures for removing sulfate from brines include ion exchangeand solvent extrac tion techniques. Each ofthese known techniques forremoving sulfate from brines involves the use of reagents and requiresspecial treating procedures. Thus, there has been a need for a physicalprocess for producing low-sulfate bischofite from sulfate-containingbrines, which requires no special chemical reagents.

SUMMARY OF THE INVENTION This invention provides a simple,straightforward process whereby brines containing both magnesiumchloride and magnesium sulfate are manipulated with respect totemperature and residence time to produce a low-sulfate bischofitecrystal product. Rather than relying on chemical methods of removingsulfate, the bischofite is actually crystallized from a brine which issupersaturated with respect to magnesium sulfate. The crystallization isconducted under carefully controlled conditions which inhibit theconcurrent crystallization of sulfate minerals.

According to this invention, magnesium chloride brines containingsulfate impurity in either minor or major amounts are evaporated toconcentrate the solution with respect to magnesium chloride. Theevaporation is done at high temperature to obtain a solution asconcentrated with respect to magnesium chloride as is practical. Brineswhich contain substantial quantities of sulfate, when treated in thisfashion, become substantially supersaturated with respect to magnesiumsulfate before the solubility limit of magnesium chloride is exceeded.lfa starting brine should contain considerably more magnesium chloridethan magnesium sulfate, it is sometimes preferable to first recovermagnesium chloride from the brine by evaporation and cooling techniquesunder conditions which retain most of the sulfate in solution.Ultimately, however, the brines become sufficiently concentrated insulfate that additional magnesium chloride crystallized therefromcontains unacceptable amounts of sulfate impurity. The brines are thenconveniently evaporated at high temperatures until they aresupersaturated with respect to magnesium sulfate and concentrated withrespect to magnesium chloride. Such brines, i.e., magnesium chloridebrines supersaturated with respect to magnesium sulfate, are treated inaccordance with the present invention to recover bischoffite low insulfate impurities, e.g., bischofite containing less than about 0.1percent by weight sulfate.

In the practice ofthis invention, a magnesium chloride brine is held ata temperature sufficiently high to promote the crystallization of MgSO5/4 H O until the concentration of magnesium sulfate in solution issubstantially reduced. During this holding procedure, the temperatureand water content of the brine are maintained sufficiently high to avoidthe crystallization of magnesium chloride. Generally it is preferred tooperate under nonconcentrating conditions, i.e., under conditions inwhich the brine loses essentially none of its water. Nonconcentratingconditions may be maintained by refluxing, if boiling temperatures areemployed. Alternatively, the brine may be held, in either an open orpressurized vessel, at a tem perature near but below its actual boilingpoint at the pressure maintained in the vessel to avoid the discharge ofvapors to the atmosphere. In any event, the brine should be held for anappropriate residence time at a temperature at least near its normalboiling point, typically above about 120 C., to promote thecrystallization of MgSO '5/4H O. The rate of crystallization of MgSO'5/4H O is higher at higher brine temperatures. Thus, it is oftenpreferred to hold the brine under elevated temperature and pressureconditions. if the brine is relatively unconcentrated with respect tomagnesium chloride when it is introduced to this holding procedure, itis sometimes useful to permit evaporation until the brine becomesconcentrated with respect to magnesium chloride.

In general, the longer the brine is held at elevated temperature, themore complete will be the crystallization of M gSO,-5 I4H O therefrom.As a practical matter, it is rarely economically justifiable to hold thebrine for the full residence time required to complete thecrystallization. It has been found, however, that completion of thecrystallization is unnecessary. Good bischofite crystal product isobtainable by cooling concentrated magnesium chloride brines which arestill supersaturated with magnesium sulfate. For purposes of thisinvention, it is normally sufficient to retain the brine at hightemperature until the concentration of MgSO, in the brine is less thanabout 20 grams per liter. Further reduction of MgSO concentration isdesirable. Thus, retention times resulting in MgSO concentrations ofless than about 16 grams per liter are preferred. Under reflux boilingat ambient pressure, residence times of about 1 to about 5 hours havebeen adequate for sulfate removal from the brine. Residence times ofless than about /2 hour are seldom satisfactory.

The residual magnesium chloride brine is separated from the MgSO .5/4 HO crystals by any suitable technique. Because these crystals are veryfine in size, clarification and decantation are usually most convenient.The temperature of clarification should be sufficiently high to retainthe magnesiurn chloride in solution.

The hot-clarified magnesium chloride brine may be treated in a varietyof ways to produce acceptable bischofite crystal crops. Variouscombinations of cooling and evaporation may be employed to obtainlow-sulfate bischofite product from brine which is still supersaturatedwith respect to magnesium sulfate. To produce a good product, however,it is necessary to conduct the crystallization at a temperaturesufficiently low and at a rate sufficiently high to avoid concurrentcrystallization of magnesium sulfate. Generally, when evaporation isrelied upon, it should be conducted under vacuum at a temperature lowerthan about C., preferably below about 80 C., and rarely above C.Evaporation may be continued until the solubility limit of magnesiumchloride in the brine is exceeded or it may be discontinued prior to theformation of a crystal crop. It is sometimes advantageous to firstevaporate under vacuum to obtain in a saturated MgCl brine, which maycontain crystallized bischofite, and then cool the brine to obtain acrystal crop. The depleted brine is then again subjected to vacuumevaporation, preferably at lower temperatures, and is again cooled toobtain a second crystal crop. In this fashion, bischofite recovery isaccomplished at reduced average temperatures and at higher rates,thereby discouraging the concurrent crystallization of magnesium sulfateminerals.

Recovery of the bischofite (MgCl '6H O) from the residual brine may beeffected by any convenient physical separation procedure. Filtration isnormally convenient, for example.

DESCRIPTION OF THE DRAWlNG The single FIGURE of the drawing is aflowsheet representing what is presently contemplated as the best modeof prac- EXAMPLE 111 The bitterns composition described in Example 1 wanvacuum evaporated at 90 C., (500-5110 mm. Hg) and "Cmgthe mvenmm' 5thereafter immediately cooled to 25 C., without first being DESCRIPTIONOF THE ILLUSTRATED EMBODIMENT held at hlgh temperature. It was thenflltered. Only a small amount 01 MgSO could be removed as soltdMgSO.5/4H,O. The invention will be better understood by reference to theThe filtrate contained 51 g./l.MgSO compared to 20g./1.Mgfollowingexamples and the drawing. 50 in Example 1 and 9g./1.MgSO in Example 11.

FXAMH Further evaporation of the filtrate in accordance with theprocedure of Example 1 produced products with 8.4 and 6.8 Referring tothe drawing, bitterns were evaporated under P 8 4 Compared 10185511111"P 8 1 vacuum (S00580 mm. Hg) to maintain a temperature of [he pmdl-lctsProduced y Exampleslandllevaporation of 90 C. until the brine wassupersaturated with I This example thus demonstrates the need forholding the magnesium sulfate. The brine was then reflux boiled in aholdbrine at a high temperature for a sufficient time period to per ingvessel for 2 hours at 1211 C. and normal atmospheric presmit thecrystallization of MgSQ, 5/4H O. sure thereby producing a slurry of MgSO-5/4H,O. TheMg- SO -5/4H O slurry was then hot-clarified at 1 C., andthe brine was recovered by decantation. Eighty percent of the 2 EXAMPLEW MgSO in the bitterns was removed in this fashion. The clarified MgClsolution was evaporated in two stages, under The fl l' omposi ion ofExample 1 was vacuum vacuum, at 85 C. and 74 C., respectively.Low-sulfate evaporated I0 3110! 460 gJ g z and was h n r x bisehofiteproduct was recovered after each stage by cooling boiled for differentlengths of time. The brine was cooled and the brine to 25 C. andfiltering. A material balance of the 25 Clarified; the clear brine wasthen analyzed. The results are reprocess is reported in Table l. portedin Table 3.

TABLE L-MATER1AL BALANQIE Grains 7 e leretmtol 'lotal, M11012, M11801,K01, NaCl, 11:0, M11012, l11 lllttcrns 4, 0158 1,127 2211 33 27 2,043100. 0

t)11t.. Evaporation: 1121) 681i 15815 MgSO1'5/4ll21) slurry:

Suit. 260 111 1711 2 1 152 1. 7 lirlno 1515b 1811 8 l 2 31511 111. 7Vacuum evaporation 1:

Steam"... 210. ..2l0. lroductl 800 3411 l) 111 421 30.7 \aeeumevaporation 11:

Steam 125.. 1215. lmductll. 335 163 0 l 1 180 13.0 Mother 1111mm... 1120300 28 l 3 5811 20.11 Loss"... 2711 121 111 111 5 1211 111.7

'Hltterns Composition: 801111111111 M14011, 12151111111111Mg1-101,1.111r1o11-s N11101:, 1.151111111111 K1011, 1,0111111101011lluU.

iipeelilc gravity at 215" (I. 1.380.

It is apparent that a similar procedure could be carried out recyclingthe mother liquor to the holding vessel. in that event, a purge streamis normally required to maintain ac- TABLE 3 S0 Removal By RefluxBoiling After ceptably low concentrations of impurities, such as lithiumand 5t) boron, in the clrculattng llquor. Vacuum Evaporation (grants perliter) Time of Reflux lloillng (minutes) Feed 11 J0 (10 )0 I Theprocedure of Example 1 was followed, starting with bit- 11". 4 4 463MHZ-6T3 V 1:* terns ofthe same composition, except that, rather thanclarify- U I'HODC th l H ldd 25C I d Ktl 11 1.4 1.4 1.4 1.4 1.4 mg .1v 1. en 1rc s urry was coo e own to an Nucl 4.0 4.2 U 4 u the solidswere liltered off. Table 2 presents a matenal balance Denllly oftheprocess: (.1 1.3311 1.3411 1.344 1.141 1.1.19 1.139

'1A11L1'12. MATERIAL BALANCE lellit'llt of Total, MgClz, M11501 1((31,Nnl l, 1110, Mgtllz, 111. 111Lter1111 4, 070 l, 220 1 14 .5, 11110 100.0

Out... llvaporatlon: 1l'1().. 51115 B115 Filtered sulfate salt:

Halt 4110 123 1113 27 15 132 10.11 15rl11e 1552 1112 4 l 2 353 17.0Vaecurn evaporation 1:

Steun1 2157 267 Product 1. 1160 3117 0 1 15 457 35. 1 Vacuum evaporation11:

Steam 210 210 Product 11. 3110 1116 0 1 l 1118 14.11 Mother liquor.11411 2114 10 1 .2 41111 111. 3 1.111111. 1111 311 22 :1 .1 M 11. 1

'lotnl 1,180 2211 114 27 Although the invention has been described withparticular reference to details of certain specific embodiments, it isnot intended thereby to limit the scope of the invention, except insofaras the details are recited in the appended claims. Many modificationswithin the scope of the invention will be suggested to those skilled inthe art by the present disclosure.

We claim: 1. The method of recovering magnesium chloride from brinescontaining both magnesium chloride and magnesium sulfate whichcomprises:

subjecting the brine to evaporation until it is substantiallysupersaturated with respect to magnesium sulfate;

holding the brine at a temperature sufficiently high to promote thecrystallization of MgSO -5/4H O until the concentration of MgSO issubstantially reduced, while maintaining the temperature and watercontent of the brine sufficiently high to avoid the crystallization ofmagnesium chloride, thereby to produce a slurry of MgSO '5 /4H Ocrystals in a concentrated magnesium chloride brine; separating thebrine from theMgSO '5/4l-l O crystals while the brine is still saturatedwith respect to MgSO and crystallizing bischofite from the said M gSO-saturated'brine at sufficiently low temperature and sufficiently highrate to avoid concurrent crystallization of further magnesium sulfate.

2. The method of claim 1, wherein after the brine is supersaturated withrespect to magnesium sulfate, it is held at a temperature at least nearits normal boiling point.

3. The method of claim 2, wherein the brine is held at said temperatureunder nonconcentrating conditions.

4. The method of claim 2, wherein the brine is held at said temperaturefor at least about one-half hour H i in? v 5. The method of claim 1,wherein the brine separated from the MgSO.,-5/4H O crystals isevaporated at a temperature below about 90 C. to crystallize bischofite.

6. The method of claim 1, whe'iiriiiiiriiiiiiiiiiii is evaporated atsufficiently high temperature that the solubility limit of magnesiumchloride is not errceeded the brine super- .ing less than about 20 gramsper liter MgSO the brine is clarified; and the clarified brine isevaporated at a temperature below about C. to crystallize bischofite.

7. The method of claim 6, wherein the supersaturated brine is held at atemperature above about C.

8. The method of claim 6, wherein the clarified brine is evaporateduntil it is saturated with respect to magnesium chloride and it is thencooled to crystallize bischofite.

9. The method of claim 8, wherein the bischofite product is recovered;the residual brine is again evaporated at a temperature below about 90C. until it is saturated with respect to magnesium chloride and thesaturated brine is again cooled to crystallize bischofite.

10. The method of recovering magnesium chloride from a magnesiumchloride brine which is supersaturated with respect to magnesium sulfatewhich comprises:

holding the brine in a vessel to permit the crystallization of MgSO'5/4l-l O while maintaining the temperature and water content of thebrine sufficiently high to avoid the crystallization of magnesiumchloride;

separating the residual brine from the MgSO -SI4H O crystals while theresidual brine is still saturated with respect to MgSO and crystallizingbischofite from the residual brine at a temperature below about 90 C.

11. The method of claim 10, wherein the initial brine is held attemperature above about 120 C. until the concentration of MgSQ, in thebrine is below about 20 grams per liter.

12. A method according to claim 10, wherein the residual brine is cooledsufficiently rapidly to avoid the concurrent crystallization ofmagnesium sulfate.

13. A method according to claim 12, wherein the cooling is effected byvacuum evaporation.

14. A method according to claim 1, wherein the brine separated from theMgSO.-5/4H O crystals is cooled to below about 90 C. to crystallizebischofite.

2. The method of claim 1, wherein after the brine is supersaturated withrespect to magnesium sulfate, it is held at a temperature at least nearits normal boiling point.
 3. The method of claim 2, wherein the brine isheld at said temperature under nonconcentrating conditions.
 4. Themethod of claim 2, wherein the brine is held at said temperature for atleast about one-half hour.
 5. The method of claim 1, wherein the brineseparated from the MgSO4.5/4H2O crystals is evaporated at a temperaturebelow about 90* C. to crystallize bischofite.
 6. The method of claim 1,wherein the initial brine is evaporated at sufficiently high temperaturethat the solubility limit of magnesium chloride is not exceeded; thebrine supersaturated with respect to magnesium sulfate is held at atemperature at least near its normal boiling point for at least aboutone-half hour until there is produced a residual brine containing lessthan about 20 grams per liter MgSO4; the brine is clarified; and theclarified brine is evaporated at a temperature below about 90* C. tocrystallize bischofite.
 7. The method of claim 6, wherein thesupersaturated brine is held at a temperature above about 120* C.
 8. Themethod of claim 6, wherein the clarified brine is evaporated until it issaturated with respect to magnesium chloride and it is then cooled tocrystallize bischofite.
 9. The method of claim 8, wherein the bischofiteproduct is recovered; the residual brine is again evaporated at atemperature below about 90* C. until it is saturated with respect tomagnesium chloride and the saturated brine is again cooled tocrystallize bischofite.
 10. The method of recovering magnesium chloridefrom a magnesium chloride brine which is supersaturated with respect tomagnesium sulfate which comprises: holding the brine in a vessel topermit the crystallization of MgSO4.5/4H2O while maintaining thetemperature and water content of the brine sufficiently high to avoidthe crystallization of magnesium chloride; separating the residual brinefrom the MgSO4.5/4H2O crystals while the residual brine is stillsaturated with respect to MgSO4; and crystallizing bischofite from theresidual brine at a temperature below about 90* C.
 11. The method ofclaim 10, wherein the initial brine is held at temperature above about120* C. until the concentration of MgSO4 in the brine is below about 20grams per liter.
 12. A method according to claim 10, wherein theresidual brine is cooled sufficiently rapidly to avoid the concurrentcrystallization of magnesium sulfate.
 13. A method according to claim12, wherein the cooling is effected by vacuum evaporation.
 14. A methodaccording to claim 1, wherein the brine separated from the MgSO4.5/4H2Ocrystals is cooled to below about 90* C. to crystallize bischofite.